1. Field of the Invention
The present invention relates to a two-stage sliding door for a machining tank of an electric discharge machine.
2. Description of the Related Art
In a typical electric discharge machine, a workpiece is fixed in a machining tank containing machining fluid and undergoes electric discharge machining in a state where the workpiece is immersed in the machining fluid. The machining tank has a lifting door on its front that moves up and down with respect to a door frame for facilitating removal of a processed workpiece from the machining tank and setting of a workpiece into the machining tank from outside. A simplest conventional structure of such a machining tank is described below with reference to FIGS. 9 to 11.
As illustrated in FIG. 9, a door 11 attached to the front of the machining tank includes a door base member 1 serving as a door frame, first and second lifting doors 2 and 3 that move up and down with respect to the door base member 1, a door cover 4 serving as a fixing cover, and a driving mechanism (not shown) for lifting and lowering the second lifting door 3. The first lifting door 2 is formed of two planar members of different heights disposed in front of each other at a certain spacing. The taller planar member has an opening formed in a center thereof, and the upper ends of the left and right portions except the center are folded forward so as to form stoppers 10. The shorter planar member has a height up to the opening in the taller planar member. The second lifting door 3 is formed of a single planar member having the same height as the shorter planar member of the first lifting door 2, and is fitted between the two planar members forming the first lifting door 2. The door base member 1 has an opening 20 formed in the center thereof. The opening 20 is not closed by the first and second lifting doors 2 and 3 when the first and second lifting doors 2 and 3 are at the lowered position, in which the second lifting door 3 is fitted between the two planar members forming the first lifting door 2, as illustrated in FIG. 9.
When the second lifting door 3 is lifted by the driving device from the position shown in FIG. 9, the second lifting door 3 gets out from between the two planar members forming the first lifting door 2 and the upper end of the second lifting door 3 abuts against the stoppers 10 of the first lifting door 2. When the second lifting door 3 is further lifted, the first lifting door 2 also moves up with the second lifting door 3 and the opening 20 on the front of the door base member 1 is closed by the first and second lifting doors 2 and 3 as shown in FIG. 11. Thus, the machining fluid in the machining tank is maintained at a high level.
With this structure, however, in order to close the opening 20 in the door base member 1 with the first and second lifting doors 2 and 3, the driving device needs to drive the first and second lifting doors 2 and 3 with the same stroke. Additionally, because load on the driving device varies when the second lifting door 3 comes into contact with the stoppers 10 of the first lifting door 2, control to keep the lifting speed of the second lifting door 3 constant is difficult. Furthermore, there is a risk of a hand of an operator present near the machining tank door 11 getting caught when the lifting second lifting door 3 comes into contact with the stoppers 10 of the first lifting door 2.
Japanese Patent Application Laid-Open No. 6-55349 discloses a machining tank of a wire electric discharge machine that includes a base container member, first and second movable container members, and an elevator for sliding the first and second movable container members in cooperation with each other up and down with respect to the base container member. With this machining tank, side walls of the base container member are partially open to a point lower than the top surface of a table on which a workpiece is secured. When the elevator is driven to slide the first and second movable container members in cooperation with each other upward with respect to the base container member, the opening portion in the side walls of the base container member is closed, forming a machining tank surrounded by high side walls on all of the four sides. When the elevator is driven to slide the first and second movable container members in cooperation with each other downward with respect to the base container member, the first and second movable container members are fitted into the base container member such that the first and second movable container members are nested. As a result, the side walls of the machining tank descend to a position lower than the top surface of the table on which a workpiece is fixed, so workpiece replacement can be easily done over the lowered side walls, i.e., removing the workpiece from the table outside the machining tank or setting a workpiece on the table. However, since a pantograph driving mechanism is employed in the elevator, which slides the first and second movable container members in cooperation with each other up and down with respect to the base container member, the structure of the elevator is complicated, leading to the problem of increase in the number of components and/or assembly time.